The present invention relates to a driving circuit for an electrochromic display device (referred to hereinafter as ECD), and more particularly, to a driving circuit for transferring electric charges for colouration from a coloured segment group to a bleached segment group (hereinafter referred to as "charge transfer driving") by the application of a voltage between the coloured segment group and the bleached segment group of the ECD.
In the prior art, the colouration/bleaching operation in an ECD is attained by the change of polarity of a voltage pulse applied between a display electrode and a counter electrode. A sectional view of the conventional ECD and a voltage waveform for driving the ECD are illustrated in FIG. 1 and FIG. 2.
In FIG. 1, a display electrode 2 which is a transparent thin film mainly consisting of In.sub.2 O.sub.3 or SnO.sub.2 is arranged on a transparent glass substrate 1, and an EC layer 3 which is a thin film mainly consisting of WO.sub.3 or MoO.sub.3 is formed on the display electrode 2.
An insulating layer 4 can be formed by evaporating Al.sub.2 O.sub.3 or the like. A counter electrode 5 is formed by evaporating Au-Cr on a glass substrate. A spacer 7 adheres to the circumferences of the transparent glass substrate and the glass substrate, and an electrolyte is sealed within the spacer.
A negative electrode of a battery 9 is connected to the display electrode 2 and a positive electrode thereof is connected to the counter electrode 5. A positive electrode of a battery 10 is connected to the transparent electrode 2 and a negative electrode of the battery 10 is connected to the counter electrode 5. Moreover, switches 11 and 12 are arranged to each battery.
According to the above-mentioned circuit construction, when the switch 11 is turned on, a negative voltage pulse is applied to the display electrode 2 to colour the EC layer 3. The condition of colouration thereof is maintained even if the switch 11 is turned off. That is, the device has a memory function.
When the switch 12 is closed, a positive voltage pulse is applied to the display electrode 2 to bleach the EC layer 3.
FIG. 2A illustrates a voltage waveform described above which has a crest value of 1.5[V] in the positive direction and the negative direction, respectively. FIG. 2B illustrates a current waveform flowing through an ECD panel.
In general, a low power dissipation is required when the ECD is incorporated in small pocketable equipments such as an electronic wrist watch, an electric calculator and the like.
In this case, the reduction of the power dissipation is attained by the use of the display method, wherein the current for the change of state between colouration and bleaching is supplied to only the segment which is changed in its display state and the previous colouration/bleaching state is maintained for the segment group which is not changed in its display state by utilizing the memory function of the ECD.
However, in the conventional driving method, the density of colouration is reduced for many number of colored segments. Therefore, the difference between the density of coloration for a segment in the memory condition and that for a segment newly coloured is produced and a non-uniform density in colouration among the segments will occur. To eliminate the non-uniformity in colouration, there is a method in which a driving condition is changed depending on the number of the coloured segments, however, the circuit for realizing the method is more complex.
Since the driving condition depends on ambient temperature, a temperature compensation circuit is required. If the temperature compensating operation is not carried out, non-uniformity in the colouration of the segments will occur.